Exchanging values of two records with unique-validated fields

[Dmitry Suzdalev]

Hi fellows!

Is there a way to atomically exchange values of records which are validated using 'validates_uniqueness_of'?

Some details follow.

Imagine I have a model:

class Item < ActiveRecord::Base

   validates_uniqueness_of :weight

end

'weight' is a sorting weight.

I have an index.html view which has a list of "Item" records wrapped in jQueryUI's sortable container.

My goal is to do some ajax request (POST i guess) whenever user changes items' order. I.e. i plan to do end up with controller action that calls function like:

class Item

  def self.exchange_weights(id1, id2)

     item1 = Item.find(id1)

     item2 = Item.find(id2)

     weight1 = Item.find(id1).weight

     weight2 = Item.find(id2).weight

     item1.weight = some_temp_weight

     item2.weight = weight1

     item1.weight = weight2

  end

end

Is this the right way to do this?

Or can I somehow just do a separate POST requests on these Item's instances by triggering ItemsController#update for each of them and passing weight - but here validations would not allow me to do this unless I miss something - that's why I desided to ask :)

Thanks.

[Peter Vandenabeele]

If you mean "atomic" on the level of saving to the database

(all or nothing), you would need a database transaction. 

http://api.rubyonrails.org/classes/ActiveRecord/Transactions/ClassMethods.html

Second, to circumvent the problem of the uniqueness validation,

I see 2 solutions:

1) what you propose above:

UNTESTED:

 def self.exchange_weights(id1, id2)

     item1 = Item.find(id1)

     item2 = Item.find(id2)

     weight1 = item1.weight

     weight2 = item2.weight

    Item.transaction do

       item1.weight = some_temp_weight

       item1.save!

       item2.weight = weight1

       item2.save!

       item1.weight = weight2

       item1.save!

    end # you still need to catch the exception here

end

This could fail ... e.g. if 2 parallel processed each write the same

some_temp_weight exactly at the same time ...

2) Alternative:

Make the validation dependent on an instance variable

UNTESTED

class Item

  attr_accessor :no_weight_uniqueness_validation

  validates :weight, :uniqueness => true, :unless => @no_weight_uniqueness_validation

end

 def self.exchange_weights(id1, id2)

     item1 = Item.find(id1)

     item2 = Item.find(id2)

     weight1 = item1.weight

     weight2 = item2.weight

    Item.transaction do

       item2.weight = weight1

       item2.no_weight_uniqueness_validation = true

       item2.save!

       item1.weight = weight2

       item1.save!

    end # you still need to catch the exception here

end

This will NOT work if you also have the uniqueness validation

in the database itself ... which you should, since the

uniqueness validation in Rails is not automatically protected

against race conditions when multiple parallell processes

write to the same database.

HTH,

Peter

--
Peter Vandenabeele
http://twitter.com/peter_v

http://rails.vandenabeele.com

[Dmitry Suzdalev]

On 5 January 2012 18:09, Peter Vandenabeele <pe...@vandenabeele.com> wrote:

If you mean "atomic" on the level of saving to the database

(all or nothing), you would need a database transaction. 

http://api.rubyonrails.org/classes/ActiveRecord/Transactions/ClassMethods.html

Second, to circumvent the problem of the uniqueness validation,

I see 2 solutions:

<snip>

Oh, it's clearer now. And, yes I have *just* saw in rails' apidocs that I should do add_index on the DB level too - to prevent possible clashes. Didn't know that until now, thanks.

I guess that I will start with the first solution then. And maybe will improve it to be the latter if find the need too.

Thanks, Peter!